We have designed a genetic test of the hypothesis that a minimum level of Myc function is required to amplify the transcriptional programs necessary for promoting and sustaining tumor formation. We employed the mouse p53 null mutant (p53 KO) as a robust tumor model to test for Myc-dependency by introducing modest changes in the endogenous c-Myc level (c-Myc+/-). Tumor-free survival times were compared in genetically similar p53 KO sibling mice that were either c-Myc+/+ (Myc WT; p53 KO), or c-Myc+/- (Myc-Het; p53 KO). Median tumor-free survival times doubled in the Myc-Het; p53 KO relative to Myc-WT (significant at P 0.0001). This difference was independent of the tumor type, with hemangiosarcoma and thymic lymphoma being the most common (83%) tumor types in both groups. Analyses of c-Myc genomic alterations and expression levels in p53 KO tumors, using Spectral Karyotyping (SKY), FISH, and quantitative RNA ISH and immunohistochemistry, revealed that compensation for the initially reduced endogenous c-Myc dosage had occurred in tumors arising in Myc-Het;p53 KO mice. Notably, in hemangiosarcomas, genome amplification achieved by several rounds of genome tetraploidization was consistently higher in the Myc-Het;p53 KO than in the Myc-WT;p53 KO tumors. Although thymic lymphomas of either genotype showed no genomic amplification, expression of Myc RNA and Myc protein were nevertheless comparably elevated in both the Myc-WT and Myc-Het tumors, suggesting that compensation for reduced Myc gene dosage had occurred at the transcriptional level. These results indicate that reduced endogenous c-Myc dosage substantially delays tumor development in mice that are genetically predisposed to neoplasia and that, in order for Myc-Het;p53 KO mice to develop tumors, a compensatory increase in expression of Myc, which can occur by multiple mechanisms, is required. We are currently testing whether reducing c-Myc will adversely impact established tumors (i.e. growth, progression) in allografts using tumors from p53 KO mice which also carry conditional c-Myc-floxed and tamoxifen-dependent Cre alleles as well as a dual-fluorescent reporter to monitor recombination. Our results strongly suggest that a modest reduction in Myc can curtail cancer growth and has important implications, particularly for extending tumor-free survival in patients with Li-Fraumeni syndrome (germline mutations in p53), as well as sporadic cancers. This study provides a framework and model for future analyses of c-Myc role in tumorigenesis and tumor progression and can be readily extended to other tumor types using analogous strategies, and other tumor promoters, such as oncogenic Ras.